The present embodiments relate to operation of a transmission device of a magnetic resonance device.
In magnetic resonance imaging, it has been proposed to configure transmission coils (e.g., radiofrequency coils) such that the transmission coils include different coil elements that may be actuated independently of one another. If it is known which magnetic fields are produced by which coil element, different excitation patterns in the imaging range may thus be produced (e.g., volume-selective excitations or the like). For this purpose and also in order to compensate for lighter or darker points in images as a result of reflections, for example, or the like, provision may be made for defined phase differences between the individual channels defined by transmission paths to be produced in order to realize interferences.
In order to realize such multichannel radiofrequency transmission systems, a corresponding transmission device has been proposed that allows independent actuation of the coil elements. The entire multichannel radiofrequency transmission system in this case may be referred to as pTX or Tx array system, where “pTX” stands for “parallel transmit”.
Overall, the distribution of the B1 field that is to be used for exciting the nuclear spin within an object to be recorded, and of the electrical field that is decisive for the SAR loading of the object may be controlled by changing the phase differences and amplitude ratios between the individual channels or transmission phases.
For the clear definition of the phase differences, a reference point for all transmission paths is to be provided. Therefore, overall, a reference plane is provided in general by the plug-in locations for the individual coil elements in the case of a magnetic resonance device. Different cable lengths to the coil elements and different properties of the coil elements may otherwise be taken into consideration since manufacturers of such radiofrequency coils and coil elements include a model of the coil elements that is taken into consideration in the actuation.
Transmission devices for multichannel radiofrequency transmission systems may have, in each transmission path, a modulator that generates the desired radiofrequency signals as low-level signal, with, in each case, one radiofrequency amplifier being connected downstream of the modulator. The radiofrequency amplifier amplifies the radiofrequency signal to the desired amplitude. The signal amplified in this way is passed on to the plug-in location of the coil. For correct actuation (e.g., in the context of magnetic resonance imaging), it is relevant that the correct input phase differences are set already at the modulators in order to obtain the correct desired target phase differences in the reference plane. In addition, reliable checking by virtue of measurement may be possible with respect to whether the correct phase differences are present. However, there are a few problems in this case.